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Influence of Al2O3 mass fractions on microstructure, oxidation resistance and friction–wear behaviors of CoCrAlYTaSi coatings
Surface & Coatings Technology 379 (2019) 125058
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Influence of Al2O3 mass fractions on microstructure, oxidation resistance and friction–wear behaviors of CoCrAlYTaSi coatings
T
Zhou Weitong, Kong Dejun∗ School of Mechanical Engineering, Changzhou University, Changzhou, 213164, China
ARTICLE INFO
ABSTRACT
Keywords: Laser cladding (LC) CoCrAlYTaSi coating Oxidation resistance Coefficient of friction (COF) Wear rate Wear mechanism
CoCrAlYTaSi coatings with different Al2O3 mass fractions were fabricated on Ti–6Al–4V alloy using a laser cladding (LC). The effects of Al2O3 mass fractions on the microstructure, oxidation resistance and friction–wear performance of obtained coatings were investigated, and the wear mechanisms were also discussed. The results show that the hardness of CoCrAlYTaSi coating increases with the increase of Al2O3 mass fractions, the dendritic structure with the Al2O3 mass fraction of 15% is compactly refined. The diffusion barrier layer of Al2O3 contains Y oxide precipitate, which reduces the oxidation rate and the movement of Ta. The total mass gain per unit area of CoCrAlYTaSi coating with the Al2O3 mass fractions of 15, 30 and 45% is 33.96, 29.82, and 26.27 mg •cm−2, respectively, the oxidation resistance with the Al2O3 mass fraction of 15% is the best among the three kinds of coatings. The average coefficients of friction (COFs) of CoCrAlYTaSi coatings with the Al2O3 mass fractions of 0, 15, 30 and 45% are 1.12, 0.17, 0.23, and 0.35, respectively, and the corresponding wear rates of are 1.59 × 10−4, 1.02 × 10−4, 0.54 × 10−4, and 0.3 × 10−4 mm3 N−1•m−1, respectively, the wear mechanism is abrasive wear and micro–cutting.
1. Introduction Ti–6Al–4V alloy is widely used in aircraft engines due to its comprehensive mechanical properties [1,2], however, its insufficient oxidation resistance and friction–wear easily causes inter–structure embrittlement [3,4], which hinders it as structural components at elevated temperatures [5]. MCrAlY (M = Ni, Co or Ni+Co) coatings is widely used on super alloy gas–turbine components at high temperatures [6], scholars at China and abroad have conducted the oxidation and friction–wear researches, which are widely used in industrial fields. With a hardness second only to diamond, Al2O3 is used as reinforcement materials in the coatings, Zhou et al. prepared the NiCrAlY/Al2O3 composite coatings, and found that the coating with the 20 wt% NiCrAlY obtained the best effective bandwidth of 1.3 GHz in the range of 8.2–9.5 GHz and the minimum reflection loss of −15.7 dB at 8.9 GHz, exhibiting the most favorable microwave absorption properties [7]; Yang et al. investigated the Al2O3–TiO2 nano–structured coatings by plasma spraying, and revealed that the nanocomposite powder with the three–dimensional network structure was composed of amorphous thin film with the intergranular network, which was enriched on the Ti, Zr and Ce surrounding the α–Al2O3 co-
∗
lonies [8]; and Li et al. analyzed the effect of Al2O3 diffusion barrier with the different thickness on the oxidation and bending properties of NiCrAlY coatings. The above investigations show that the Al2O3 diffusion barrier with the various thicknesses delayed the element diffusion and improve the oxidation properties [9], literature is focused on the effects of Al2O3 addition on the oxidation and friction–wear of Ni–based coatings. However, there are few reports about the effects of Al2O3 on the oxidation and friction–wear of Co–based coatings at high temperatures [10,11], which becomes a difficult problem in its applications. In this study, CoCrAlYTaSi coatings with the different mass fractions of Al2O3 were fabricated on Ti–6Al–4V alloy. The aim was to investigate the effects of Al2O3 mass fractions on the oxidation and friction–wear behaviors of obtained coatings at high temperature, which lengthened the service life of Ti–6Al–4V alloy. 2. Experimental procedures 2.1. Sample fabrications The substrate was Ti–6Al–4V alloy with the nominal composition
Corresponding author. Tel.: 86 051981169812; fax: 86 051981169810. E-mail address: [email protected] (K. Dejun).
https://doi.org/10.1016/j.surfcoat.2019.125058 Received 11 August 2019; Received in revised form 22 September 2019; Accepted 8 October 2019 Available online 09 October 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
Surface & Coatings Technology 379 (2019) 125058
Z. Weitong and K. Dejun
Fig. 1. Schematic diagrams of LC and friction–wear processes.
(wt, %): Al 5.5–6.75, V 3.5–4.5, the rest was Ti. The substrate surface was grit blasted with 80 mesh alumina grit, cleaned and removed grease by ethyl alcohol cleaner, and then dried and pre–heated at 200 °C. The commercial powders of CoCrAlYTaSi and Al2O3 were used as the original materials, among them, the chemical composition of CoCrAlYTaSi powder (wt, %): Co 36.24, Cr 22.53, Al 18.46, Y 2.16, Ta 14.33 and Si 6.25, O 0.03. The respective mass fraction ratios of CoCrAlYTaSi and Al2O3 powders were 85%:15%, 70%:30% and 55%:45%, which were milled using a high–energy planetary ball miller for 300 min with the ball–powder mass fraction ratio of 80%:10% and the speed of 250 rpm. The LC test performed on a ZKSX–2008 type laser spraying system, the powder was delivered synchronously using a DPSF–3 type powder feeder, Ar gas with the purity of 99% was used for protection, as shown in Fig. 1 (a). Technique parameters: power of 1000 W, scanning speed of 3 mm •s−1, spot diameter of 5 mm, Ar gas speed of 5 L •min−1, powder speed of 8 g •min−1.
digital microscope system. The wear rate was [16].
K=
V S•F
(1) 3
where V was the wear volume/mm ; S was the total sliding distance/m; and F was the wear normal load/N. 3. Analysis and discussion 3.1. Morphologies, EDS and XRD analysis of powders Fig. 2 (a) shows the morphology and EDS result of CoCrAlYTaSi powder with the Al2O3 mass fraction of 15%. The Al2O3 with the dark–black shape was obviously found in the mixed powders, the mass fractions of chemical elements (mass, %) were Co 46.45, Cr 23.57, Al 14.55, Y 0.72, Ta 10.84, Si 0.50 and O 3.37, which was the constituent elements of powders. Fig. 2 (b) shows the morphology and EDS result of CoCrAlYTaSi powder with the Al2O3 mass fraction of 30%. The same characteristic of Al2O3 was also observed in mixed powders, the mass fractions of chemical elements (mass, %) were Co 36.81, Cr 19.83, Al 28.14, Y 0.06, Ta 11.68, Si 0.48 and O 3.00, with no other impurity elements. Fig. 2 (c) shows the morphology and EDS result of CoCrAlYTaSi powder with the Al2O3 mass fraction of 45%. The Al2O3 appeared in the mixed powders, the mass fractions of chemical elements (mass, %) were Co 17.14, Cr 14.58, Al 48.55, Ta 12.89, Si 0.40 and O 6.44, Y was not detected due to its low content, Ta accounted for 12.89%, which promoted the bonding strength between the coating and the substrate [17]. Fig. 3 shows the XRD patterns of CoCrAlYTaSi powders with the different Al2O3 mass fractions. The peaks of CoO, Al2O3, AlCo, AlCr2, TaO, Co2Ta and Al1.92Cr0.08O3 were detected in the mixed powders, among them, the peaks of CoO and TaO were caused by the oxidation of alloy elements in the air, and the strong Al2O3 peak meant that the agglomeration facilitated the well dispersion of Al2O3 particles in the to–be–sprayed powders [18].
2.2. Characterization methods The morphologies and chemical elements of obtained powders and coatings were analyzed using a JSM–6360LA type scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS), respectively, and the phases were measured using a D/max 2500 PC type X–ray diffraction (XRD) in the range of 20–90°. The hardness was measured using a HMV–2T type Vickers hardness tester with the load of 200 N and the hold time of 12 s, and the tested points were distributed at 250 μm intervals on the across–section. The oxidation test was conducted by thermo gravimetric analysis (TGA), the CoCrAlYTaSi coatings with the different Al2O3 mass fractions were heated in 25–1000 °C with the rate of 7 °C/min to evaluate the oxidation performance. The friction–wear test was carried out on a HT–1000 type high temperature friction–wear tester, the COFs were self–recorded by a computer, as shown in Fig. 1 (b). According to the literature [12–15], 800 °C was selected to conduct the friction-wear test in this case, Si3N4 ball with the hardness of 77 HRC was selected as the tribo-pair on the CoCrAlYTaSi coating with the hardness of 873 HV. Test parameters: wear load of 3 N, wear distance of 120 m, rotation radius of 4 mm. The morphologies, chemical elements and phases of worn tracks were analyzed using a SEM, EDS, and XRD, respectively, and the wear profiles were measured using a VHX–700FC type three–dimensional
3.2. Morphologies and XRD analysis of coating surfaces Fig. 4 (a) shows the morphology of CoCrAlYTaSi coating surface with the Al2O3 mass fraction of 15%. The coating presented the dense structure, with no evident micro–defects. The black dendritic structure existed, this was because the rapid condensation of sprayed powders
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Fig. 2. Morphologies and EDS results of CoCrAlYTaSi powders with different Al2O3 mass fractions.
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molten pool began to condense rapidly, the temperature of substrate was lower than that in the molten pool, the cooling speed was fast in the direction perpendicular to the substrate, and the dendrite growth direction was also perpendicular to the substrate. Fig. 6 (b) shows the morphology of CoCrAlYTaSi coating cross–section with the Al2O3 mass fraction of 30%. The white bright band at the junction line became disordered, which was the influence of flow characteristics of molten alloys and the interaction between the liquid and solid interfaces. The Al2O3 formed the enriched zones on the middle and bottom parts of cross–section, the cooling and heating transfer of substrate became disordered due to strong convection when passing through the enriched zones of Al2O3 [21]. Fig. 6 (c) shows the morphology of CoCrAlYTaSi coating cross–section with the Al2O3 mass fraction of 45%. There were pits at the junction line, no sign of martensite was observed, this was because the Al2O3 was deposited at the bottom of molten pool and formed the enriched zones at the junction line between the coating and the substrate, which increased the coating brittleness. The addition of Al2O3 caused the great undercooling at the front of dendritic interface, the growth of dendrites was limited by the promotion of liquid metal forming core, the dendrites become small and dense to refine the microstructure of CoCrAlYTaSi coating [22].
Fig. 3. XRD patterns of CoCrAlYTaSi powders with different Al2O3 mass fractions.
and the formation of isothermal difference in the direction were perpendicular to the substrate in the LC process. Fig. 4 (b) shows the morphology of CoCrAlYTaSi coating surface with the Al2O3 mass fraction of 30%. The decrease of black dendritic structure was observed on the coating, and the porosity of 8.5% was measured with the image analyzer. Fig. 4 (c) shows the morphology of CoCrAlYTaSi coating surface with the Al2O3 mass fraction of 45%. The porosity increased to 22.5%, this was the LC induced gas was not escaped in time, which produced the porosity in the process of rapid cooling, the deposition of Al2O3 accelerated the cooling rate, as a result, the porosity increased with the increase of Al2O3 mass fractions. Fig. 5 shows the XRD patterns of CoCrAlYTaSi coatings with the different Al2O3 mass fractions. The peaks of CoO, Al2O3, AlCo, SiC, Cr7C3, Cr23C6 and TaC were detected on the coating, suggesting that the powder peaks of CoO, Al2O3 and AlCo retained on the coating, and the hard phases Cr7C3, Cr23C6 and TaC were also detected at 36.5, 43.6 and 61.8o, respectively, which increased with the increase of Al2O3 mass fractions. The Cr and Ta existed in the form of carbide hard phases, in which the Cr7C3 improved the hardness and wear resistance [19]. The peak of α–Al2O3 was detected at 42.5°, indicating that the structure of Al2O3 did not change due to the small thermal radiation of LC. From the above XRD analyses, it can be seen that the peaks of CoTi and TiO were detected, showing that the Ti of substrate was diffused into the coating and formed the new compound of CoTi with the Co [20], and the diffusion bonding was formed at the coating-substrate interface. Fig. 6 (a) shows the morphology of CoCrAlYTaSi coating cross–section with the Al2O3 mass fraction of 15%. The cross–section was composed of coating, heat–affected zone and substrate, there was a white bright structure perpendicular to the substrate on the heat affected zone, this was because the substrate temperature was higher than the critical point. When the laser irradiation position changed, the martensite structure was formed on the heat–affected zone by heat transfer quenching of substrate, indicating that the metallurgical bonding was obtained between the coating and the substrate. There were obvious characteristics of black dendrite growth with the trend of vertical bonding line in the direction of dendrite growth, i.e., along the direction of heat transfer. This was because the alloy compounds in the
3.3. Hardness distributions Fig. 7 shows the hardness of CoCrAlYTaSi coating cross–sections with the different Al2O3 mass fractions. The hardness of coatings increased with the increase of Al2O3 mass fractions, there was a phenomenon that the hardness increased first at the curve fronts, this was because the surface temperature of molten pool was high, the elements on the coating surface were damaged by burning out, the hardness of surface layer with the dendritic structure was low. The average hardness of coatings with the Al2O3 mass fractions of 15, 30 and 45% was 1133, 1249, and 1322 HV0.2, respectively, which increased with the increase of Al2O3 mass fractions, the enriched zones of Al2O3 played a major role in the strengthened coatings. 3.4. Oxidation resistance TGA was used to investigate the oxidation behaviors of CoCrAlYTaSi coatings with the different Al2O3 mass fractions. Fig. 8 shows the results of mass gain per unit as the temperature from 400 up to 600 °C. The total mass gain per unit area of coatings with the Al2O3 mass fractions of 15, 30 and 45% were 33.96, 29.82, and 26.27 mg •cm−2, respectively, indicating that the coatings had better oxidation resistance at the range of 400–1000 °C. The mass gain per unit area of coatings with the Al2O3 mass fraction of 15% kept the fast increase in the heating process, while that of coatings with the mass fraction of 45% increased slowly, suggesting that the oxidation resistance enhanced with the increase of Al2O3 mass fractions. The diffusion coefficient of Al was higher than those of other elements at the grain boundary of Al2O3, which improved the oxidation resistance of coatings. Fig. 9 shows the morphologies of CoCrAlYTaSi coating cross–sections with the different Al2O3 mass fractions after the oxidation test. The oxide layers with the gray-white were dispersed on the cross–sections with the Al2O3 mass fraction of 15 and 30%, the cracks existed on the oxide layers, which were diffused from the bonding line to the substrate; while the cross–section with the Al2O3 mass fraction of 45% was compact and regular plate structure. Research reported that the Al
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Fig. 4. Morphologies of CoCrAlYTaSi coating surfaces with different Al2O3 mass fractions.
was high enough to form the Al layer at the oxidation beginning [23], however, the rapid consumption of Al led to the decrease of Al at the bonding interface, the Al2O3 reduced to below the critical level. At this time, the spinel oxides of Co, Cr and Y and the Ta-riched oxide through the Al2O3 layer, prevented and slowed down the oxide growth of CoCrAlYTaSi coating with the Al2O3 mass fraction of 45% [24].
3.5. Friction–wear behaviors 3.5.1. COFs and wear rates Fig. 10 (a) shows the COFs of CoCrAlYTaSi coatings with the different Al2O3 mass fractions vs wear time. The average COFs of CoCrAlYTaSi coating with the Al2O3 mass fractions of 0% was 1.12, while those of CoCrAlYTaSi coatings with the Al2O3 mass fractions of 15, 30
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3.5.2. XRD analysis and wear mechanism Fig. 12 (a) shows the XRD analysis of worn track on the CoCrAlYTaSi coating with the Al2O3 mass fraction of 15%. The CoO peak was detected at 36.68 and 42.346°, and the α–Al2O3 peak appeared at 43.173 o, with no γ–Al2O3, this was because the γ–Al2O3 was transformed into the α–Al2O3 after the wear test [32]. The Ta2O5 and Cr2O3 oxides were detected at 36.914°, and the Ti oxide was detected at 42.611°, which was diffused from the substrate into the coating. The above oxides were mainly produced by the wear debris in the tribochemical reaction, when the content of fragments reached a certain value, they were sintered and fixed on the worn track, acting as a lubricant in the wear process to reduce the COFs. The phase of Al9Co2 presented a widened peak, indicating that the grain size was refined. In additional, the TaC, Al13Co4, Cr3C2 and phases were also detected on the worn track. Fig. 12 (b) shows the XRD analysis of worn track on the CoCrAlYTaSi coating with the Al2O3 mass fraction of 30%. The oxides of CoO, Ta2O5 and Ti were detected on the worn track, the crystallization peaks of CoO and Ta2O5 decreased; while that of Al2O3 peak increased. As a result, the thickness of oxide films sintered decreased, more Al2O3 was distributed on the worn track, the oxide films were still acted as a lubricant to play an anti–wear role [33]. Fig. 12 (c) shows the XRD analysis of worn track on the CoCrAlYTaSi/Al2O3 coating with the Al2O3 mass fraction of 45%. The crystalline peaks of CoO and Ta2O5 further decreased due to the reduction of CoCrAlYTaSi mass fraction, only a thin oxide film was formed on the worn track, the hard phases of Al2O3 and Cr3C2 played a dominant role, the COFs were the largest among the three kind of coatings, the hard phase was the main factor of improving the wear resistance [34]. Fig. 13 (a) shows the morphologies of worn tracks on the coating with the Al2O3 mass fraction of 15%. The scratches and parallel furrows were observed on the worn track, the dominant characteristic was adhesive and delamination wear. After further enlarging the plough groove, it can be seen that there was the serious tearing phenomenon along the sliding direction, the debris did not fall off from the coating and continued to wear on the scratched surface, the wear mechanism was abrasive wear. This was because the shearing stress was less than the bonding strength of coating, the shearing failure occurred on the worn track with the wide and deep scratches. Remarkably, the mixed oxides of CoO, Cr2O3 and Ta2O5 oxides were detected on the worn tracks, which played a role of lubrication to reduce the COFs and wear rates. Among them, the Cr2O3 was devoted to enhance the wear performance due to its inherent hardness and chemical stability [35], the wear mechanism of CoCrAlYTaSi coating with the Al2O3 mass fraction of 15% was transited from the adhesive and delamination wear to abrasive and tribo-oxidation wear [36]. Fig. 13 (b) shows the morphologies of worn tracks on the coating with the Al2O3 mass fraction of 30%. The fine abrasive scratches were observed on the smooth worn track, the wear mechanism was micro–cutting, showing the higher wear resistance than the coating with the Al2O3 mass fraction of 15%. The wear rate was [37].
Fig. 5. XRD patterns of CoCrAlYTaSi coatings with different Al2O3 mass fractions.
and 45% were 0.17, 0.23, and 0.35, respectively, decreased by 85, 79 and 69% than the CoCrAlYTaSi coating, respectively. The COFs increased with the increase of Al2O3 mass fractions, this was because the separation of Al2O3 hard phase felled on the worn track under the action of tribo–pair, with no additive liquid and solid lubricants, the average COFs were less than 0.4. Fig. 10 (b) shows the profile curves of worn tracks on the CoCrAlYTaSi coatings with different Al2O3 mass fractions. The depths of worn track with the Al2O3 mass fractions of 0, 15, 30 and 45% were 12, 3.83, 5.53, and 5.92 μm, respectively, and the corresponding wear rates were 1.59 × 10−4, 1.02 × 10−4, 0.54 × 10−4, and 0.3 × 10−4 mm3/N • m, respectively. The profile curves of CoCrAlYTaSi coating with the different Al2O3 mass fractions were more fluctuant than that of CoCrAlYTaSi coating, which indicated that the wear resistance was improved by the addition of Al2O3. The wear rate of CoCrAlYTaSi coating with the Al2O3 mass fractions of 15% was similar to that of laser cladded NiCoCrAlY coating under the same environment [25]; while those with the Al2O3 mass fractions of 30 and 45% were lower than 0.96 × 10−4 mm3 N−1•m−1 of HVOF sprayed NiCoCrAlY–Al2O3 coating [26]. It was remarkable that the COFs of CoCrAlYTaSi coatings with the different Al2O3 mass fractions were significantly lowed than that of CoCrAlYTaSi coating. The oxides appeared on the worn track, which reduced the COFs [27], as shown in Fig. 11. The Al2O3 was the main component of debris on the worn track in the normal wear period, the COFs were stable due to the lubrication of Al2O3, which reduced the friction force between the coatings and the tribo-pair [28,29]. This was also the reason why the CoCrAlYTaSi coating with the different Al2O3 mass fractions took less time than the CoCrAlYTaSi coating from the running-in to normal wear periods [30]. The COFs of CoCrAlYTaSi coating increased with the increase of Al2O3 mass fractions, exhibiting the friction performance of coating with the Al2O3 mass fractions of 15% was the best among the three kinds of coatings. The phases of CoO and TaO played an important role in the formation of continuous lubrication film on the worn tracks [31], which decreased with the increase of Al2O3 mass fractions.
W∝(KIC)−3/4 ×H−1/2
(2)
where KIC was the fracture toughness; and H was the micro-hardness of coatings. From Eq. (2), it can been known the wear resistance improved with
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Fig. 6. Morphologies of CoCrAlYTaSi coating cross–sections with different Al2O3 mass fractions.
the increase of hardness. After adding more Al2O3 content, the abrasive wear was improved due to its higher hardness, which prevented the plastic deformation and adhesion wear occur on the worn tracks. Fig. 13 (c) shows the morphologies of worn tracks on the coating with the Al2O3 mass fraction of 45%. There were layered and scaly features on the worn track, suggesting that the coating was exfoliated and fractured, which originated from the Ti diffusion in the substrate,
the thermal stress at the coating-substrate interface was the main factor of mechanical damage [38]. In this case, the plastic deformation of coating surface was introduced into the grain interior, which resulted in the tensile stress increasing and accumulating along the grain boundary and. In multiphase composites consisting of CoCrAlYTaSi and Al2O3, the internal residual stress was mainly caused by the following thermal expansion mismatch.
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The stress intensity factor was K(t)=Y(σD(t)+q)bd1/2
(4)
where Y and b were the crack geometry parameters; σD(t) was the tensile stress during the plastic deformation damage; q was the thermal expansion mismatch; d was the grain size [39]. From Eq. (4), the increase rate of stress intensity factor K(t) was faster than that of thermal expansion mismatch q with the increase of Al2O3 mass fractions. This was because the increase of Al2O3 hard phase at the interface decreased the metallurgical bonding strength between the coating and the substrate, which led to the brittleness increase of coating. By magnifying the fracture surface, it was found that the morphology presented the brittle fracture with the relatively flat, only a small amount of micro–abrasive grinding. It was included that the increase of Al2O3 mass fraction improved the abrasive resistance, the brittle fracture produced the micro–grinding effect on the worn track, and the wear resistance of coating with the Al2O3 mass fraction of 45% was excellent. However, the coating easily broken and fell off to increase the COFs with the increase of Al2O3 mass fractions [40], which reduced its friction performance. From the above analyses, it can be seen that the wear mechanisms of CoCrAlYTaSi coatings with the Al2O3 mass fractions of 15 and 30% were abrasive wear and micro–cutting, respectively; while that with the Al2O3 mass fraction of 45% was micro–abrasive wear, the main factor was the increase of the Al2O3 mass fractions to reduce the wear degree.
Fig. 7. Hardness of CoCrAlYTaSi coating cross–sections with different Al2O3 mass fractions.
4. Conclusions (1) The dendritic structure of CoCrAlYTaSi coating with the Al2O3 mass fraction of 15% is further refined, which grows perpendicular to the substrate, the metallurgical bonding is formed at the coating interface. (2) The hardness of CoCrAlYTaSi coatings the Al2O3 mass fractions of 15, 30 and 45% is 1133, 1249, and 1322 HV0.2, respectively, which increases with the increase of Al2O3 mass fractions. (3) The total mass gains per unit area of CoCrAlYTaSi coatings with the Al2O3 mass fractions of 15, 30 and 45% is 33.96, 29.82, and 26.27 mg/cm2, respectively, the oxidation resistance in the range of 400–1000 °C is excellent. (4) The average COFs of CoCrAlYTaSi coatings with the Al2O3 mass fractions of 0, 15, 30 and 45% are 1.12, 0.17, 0.23, and 0.35, respectively, and the corresponding wear rates are 1.02 × 10−4, 0.54 × 10−4, and 0.3 × 10−4 mm3/N•m, respectively. The wear mechanism is abrasive wear and micro–cutting, the wear reduction is the best among the three kinds of coatings.
Fig. 8. Mass gain per unit area of CoCrAlYTaSi coatings with different Al2O3 mass fractions in range of 400–1000 °C.
q = V(1-V)E△T△α
(3)
where V was the volume fraction of second phase Al2O3; E was the average elastic modulus; △T was the range of temperature; and △α was the thermal expansion mismatch.
Fig. 9. Microstructure of CoCrAlYTaSi coating cross–sections with different Al2O3 mass fractions after oxidation test.
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Fig. 10. COFs vs wear time and profiles of worn tracks on CoCrAlYTaSi coatings with different Al2O3 mass fractions.
Fig. 11. Wear model of CoCrAlYTaSi coatings with different Al2O3 mass fractions wear.
Fig. 12. XRD analysis of worn tracks on CoCrAlYTaSi coatings with different Al2O3 mass fractions.
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Fig. 13. Morphologies of wear tracks on CoCrAlYTaSi coatings with different Al2O3 mass fractions.
Acknowledgements
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Influence of Al2O3 mass fractions on microstructure, oxidation resistance and friction–wear behaviors of CoCrAlYTaSi coatings
T
Zhou Weitong, Kong Dejun∗ School of Mechanical Engineering, Changzhou University, Changzhou, 213164, China
ARTICLE INFO
ABSTRACT
Keywords: Laser cladding (LC) CoCrAlYTaSi coating Oxidation resistance Coefficient of friction (COF) Wear rate Wear mechanism
CoCrAlYTaSi coatings with different Al2O3 mass fractions were fabricated on Ti–6Al–4V alloy using a laser cladding (LC). The effects of Al2O3 mass fractions on the microstructure, oxidation resistance and friction–wear performance of obtained coatings were investigated, and the wear mechanisms were also discussed. The results show that the hardness of CoCrAlYTaSi coating increases with the increase of Al2O3 mass fractions, the dendritic structure with the Al2O3 mass fraction of 15% is compactly refined. The diffusion barrier layer of Al2O3 contains Y oxide precipitate, which reduces the oxidation rate and the movement of Ta. The total mass gain per unit area of CoCrAlYTaSi coating with the Al2O3 mass fractions of 15, 30 and 45% is 33.96, 29.82, and 26.27 mg •cm−2, respectively, the oxidation resistance with the Al2O3 mass fraction of 15% is the best among the three kinds of coatings. The average coefficients of friction (COFs) of CoCrAlYTaSi coatings with the Al2O3 mass fractions of 0, 15, 30 and 45% are 1.12, 0.17, 0.23, and 0.35, respectively, and the corresponding wear rates of are 1.59 × 10−4, 1.02 × 10−4, 0.54 × 10−4, and 0.3 × 10−4 mm3 N−1•m−1, respectively, the wear mechanism is abrasive wear and micro–cutting.
1. Introduction Ti–6Al–4V alloy is widely used in aircraft engines due to its comprehensive mechanical properties [1,2], however, its insufficient oxidation resistance and friction–wear easily causes inter–structure embrittlement [3,4], which hinders it as structural components at elevated temperatures [5]. MCrAlY (M = Ni, Co or Ni+Co) coatings is widely used on super alloy gas–turbine components at high temperatures [6], scholars at China and abroad have conducted the oxidation and friction–wear researches, which are widely used in industrial fields. With a hardness second only to diamond, Al2O3 is used as reinforcement materials in the coatings, Zhou et al. prepared the NiCrAlY/Al2O3 composite coatings, and found that the coating with the 20 wt% NiCrAlY obtained the best effective bandwidth of 1.3 GHz in the range of 8.2–9.5 GHz and the minimum reflection loss of −15.7 dB at 8.9 GHz, exhibiting the most favorable microwave absorption properties [7]; Yang et al. investigated the Al2O3–TiO2 nano–structured coatings by plasma spraying, and revealed that the nanocomposite powder with the three–dimensional network structure was composed of amorphous thin film with the intergranular network, which was enriched on the Ti, Zr and Ce surrounding the α–Al2O3 co-
∗
lonies [8]; and Li et al. analyzed the effect of Al2O3 diffusion barrier with the different thickness on the oxidation and bending properties of NiCrAlY coatings. The above investigations show that the Al2O3 diffusion barrier with the various thicknesses delayed the element diffusion and improve the oxidation properties [9], literature is focused on the effects of Al2O3 addition on the oxidation and friction–wear of Ni–based coatings. However, there are few reports about the effects of Al2O3 on the oxidation and friction–wear of Co–based coatings at high temperatures [10,11], which becomes a difficult problem in its applications. In this study, CoCrAlYTaSi coatings with the different mass fractions of Al2O3 were fabricated on Ti–6Al–4V alloy. The aim was to investigate the effects of Al2O3 mass fractions on the oxidation and friction–wear behaviors of obtained coatings at high temperature, which lengthened the service life of Ti–6Al–4V alloy. 2. Experimental procedures 2.1. Sample fabrications The substrate was Ti–6Al–4V alloy with the nominal composition
Corresponding author. Tel.: 86 051981169812; fax: 86 051981169810. E-mail address: [email protected] (K. Dejun).
https://doi.org/10.1016/j.surfcoat.2019.125058 Received 11 August 2019; Received in revised form 22 September 2019; Accepted 8 October 2019 Available online 09 October 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
Surface & Coatings Technology 379 (2019) 125058
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Fig. 1. Schematic diagrams of LC and friction–wear processes.
(wt, %): Al 5.5–6.75, V 3.5–4.5, the rest was Ti. The substrate surface was grit blasted with 80 mesh alumina grit, cleaned and removed grease by ethyl alcohol cleaner, and then dried and pre–heated at 200 °C. The commercial powders of CoCrAlYTaSi and Al2O3 were used as the original materials, among them, the chemical composition of CoCrAlYTaSi powder (wt, %): Co 36.24, Cr 22.53, Al 18.46, Y 2.16, Ta 14.33 and Si 6.25, O 0.03. The respective mass fraction ratios of CoCrAlYTaSi and Al2O3 powders were 85%:15%, 70%:30% and 55%:45%, which were milled using a high–energy planetary ball miller for 300 min with the ball–powder mass fraction ratio of 80%:10% and the speed of 250 rpm. The LC test performed on a ZKSX–2008 type laser spraying system, the powder was delivered synchronously using a DPSF–3 type powder feeder, Ar gas with the purity of 99% was used for protection, as shown in Fig. 1 (a). Technique parameters: power of 1000 W, scanning speed of 3 mm •s−1, spot diameter of 5 mm, Ar gas speed of 5 L •min−1, powder speed of 8 g •min−1.
digital microscope system. The wear rate was [16].
K=
V S•F
(1) 3
where V was the wear volume/mm ; S was the total sliding distance/m; and F was the wear normal load/N. 3. Analysis and discussion 3.1. Morphologies, EDS and XRD analysis of powders Fig. 2 (a) shows the morphology and EDS result of CoCrAlYTaSi powder with the Al2O3 mass fraction of 15%. The Al2O3 with the dark–black shape was obviously found in the mixed powders, the mass fractions of chemical elements (mass, %) were Co 46.45, Cr 23.57, Al 14.55, Y 0.72, Ta 10.84, Si 0.50 and O 3.37, which was the constituent elements of powders. Fig. 2 (b) shows the morphology and EDS result of CoCrAlYTaSi powder with the Al2O3 mass fraction of 30%. The same characteristic of Al2O3 was also observed in mixed powders, the mass fractions of chemical elements (mass, %) were Co 36.81, Cr 19.83, Al 28.14, Y 0.06, Ta 11.68, Si 0.48 and O 3.00, with no other impurity elements. Fig. 2 (c) shows the morphology and EDS result of CoCrAlYTaSi powder with the Al2O3 mass fraction of 45%. The Al2O3 appeared in the mixed powders, the mass fractions of chemical elements (mass, %) were Co 17.14, Cr 14.58, Al 48.55, Ta 12.89, Si 0.40 and O 6.44, Y was not detected due to its low content, Ta accounted for 12.89%, which promoted the bonding strength between the coating and the substrate [17]. Fig. 3 shows the XRD patterns of CoCrAlYTaSi powders with the different Al2O3 mass fractions. The peaks of CoO, Al2O3, AlCo, AlCr2, TaO, Co2Ta and Al1.92Cr0.08O3 were detected in the mixed powders, among them, the peaks of CoO and TaO were caused by the oxidation of alloy elements in the air, and the strong Al2O3 peak meant that the agglomeration facilitated the well dispersion of Al2O3 particles in the to–be–sprayed powders [18].
2.2. Characterization methods The morphologies and chemical elements of obtained powders and coatings were analyzed using a JSM–6360LA type scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS), respectively, and the phases were measured using a D/max 2500 PC type X–ray diffraction (XRD) in the range of 20–90°. The hardness was measured using a HMV–2T type Vickers hardness tester with the load of 200 N and the hold time of 12 s, and the tested points were distributed at 250 μm intervals on the across–section. The oxidation test was conducted by thermo gravimetric analysis (TGA), the CoCrAlYTaSi coatings with the different Al2O3 mass fractions were heated in 25–1000 °C with the rate of 7 °C/min to evaluate the oxidation performance. The friction–wear test was carried out on a HT–1000 type high temperature friction–wear tester, the COFs were self–recorded by a computer, as shown in Fig. 1 (b). According to the literature [12–15], 800 °C was selected to conduct the friction-wear test in this case, Si3N4 ball with the hardness of 77 HRC was selected as the tribo-pair on the CoCrAlYTaSi coating with the hardness of 873 HV. Test parameters: wear load of 3 N, wear distance of 120 m, rotation radius of 4 mm. The morphologies, chemical elements and phases of worn tracks were analyzed using a SEM, EDS, and XRD, respectively, and the wear profiles were measured using a VHX–700FC type three–dimensional
3.2. Morphologies and XRD analysis of coating surfaces Fig. 4 (a) shows the morphology of CoCrAlYTaSi coating surface with the Al2O3 mass fraction of 15%. The coating presented the dense structure, with no evident micro–defects. The black dendritic structure existed, this was because the rapid condensation of sprayed powders
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Fig. 2. Morphologies and EDS results of CoCrAlYTaSi powders with different Al2O3 mass fractions.
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molten pool began to condense rapidly, the temperature of substrate was lower than that in the molten pool, the cooling speed was fast in the direction perpendicular to the substrate, and the dendrite growth direction was also perpendicular to the substrate. Fig. 6 (b) shows the morphology of CoCrAlYTaSi coating cross–section with the Al2O3 mass fraction of 30%. The white bright band at the junction line became disordered, which was the influence of flow characteristics of molten alloys and the interaction between the liquid and solid interfaces. The Al2O3 formed the enriched zones on the middle and bottom parts of cross–section, the cooling and heating transfer of substrate became disordered due to strong convection when passing through the enriched zones of Al2O3 [21]. Fig. 6 (c) shows the morphology of CoCrAlYTaSi coating cross–section with the Al2O3 mass fraction of 45%. There were pits at the junction line, no sign of martensite was observed, this was because the Al2O3 was deposited at the bottom of molten pool and formed the enriched zones at the junction line between the coating and the substrate, which increased the coating brittleness. The addition of Al2O3 caused the great undercooling at the front of dendritic interface, the growth of dendrites was limited by the promotion of liquid metal forming core, the dendrites become small and dense to refine the microstructure of CoCrAlYTaSi coating [22].
Fig. 3. XRD patterns of CoCrAlYTaSi powders with different Al2O3 mass fractions.
and the formation of isothermal difference in the direction were perpendicular to the substrate in the LC process. Fig. 4 (b) shows the morphology of CoCrAlYTaSi coating surface with the Al2O3 mass fraction of 30%. The decrease of black dendritic structure was observed on the coating, and the porosity of 8.5% was measured with the image analyzer. Fig. 4 (c) shows the morphology of CoCrAlYTaSi coating surface with the Al2O3 mass fraction of 45%. The porosity increased to 22.5%, this was the LC induced gas was not escaped in time, which produced the porosity in the process of rapid cooling, the deposition of Al2O3 accelerated the cooling rate, as a result, the porosity increased with the increase of Al2O3 mass fractions. Fig. 5 shows the XRD patterns of CoCrAlYTaSi coatings with the different Al2O3 mass fractions. The peaks of CoO, Al2O3, AlCo, SiC, Cr7C3, Cr23C6 and TaC were detected on the coating, suggesting that the powder peaks of CoO, Al2O3 and AlCo retained on the coating, and the hard phases Cr7C3, Cr23C6 and TaC were also detected at 36.5, 43.6 and 61.8o, respectively, which increased with the increase of Al2O3 mass fractions. The Cr and Ta existed in the form of carbide hard phases, in which the Cr7C3 improved the hardness and wear resistance [19]. The peak of α–Al2O3 was detected at 42.5°, indicating that the structure of Al2O3 did not change due to the small thermal radiation of LC. From the above XRD analyses, it can be seen that the peaks of CoTi and TiO were detected, showing that the Ti of substrate was diffused into the coating and formed the new compound of CoTi with the Co [20], and the diffusion bonding was formed at the coating-substrate interface. Fig. 6 (a) shows the morphology of CoCrAlYTaSi coating cross–section with the Al2O3 mass fraction of 15%. The cross–section was composed of coating, heat–affected zone and substrate, there was a white bright structure perpendicular to the substrate on the heat affected zone, this was because the substrate temperature was higher than the critical point. When the laser irradiation position changed, the martensite structure was formed on the heat–affected zone by heat transfer quenching of substrate, indicating that the metallurgical bonding was obtained between the coating and the substrate. There were obvious characteristics of black dendrite growth with the trend of vertical bonding line in the direction of dendrite growth, i.e., along the direction of heat transfer. This was because the alloy compounds in the
3.3. Hardness distributions Fig. 7 shows the hardness of CoCrAlYTaSi coating cross–sections with the different Al2O3 mass fractions. The hardness of coatings increased with the increase of Al2O3 mass fractions, there was a phenomenon that the hardness increased first at the curve fronts, this was because the surface temperature of molten pool was high, the elements on the coating surface were damaged by burning out, the hardness of surface layer with the dendritic structure was low. The average hardness of coatings with the Al2O3 mass fractions of 15, 30 and 45% was 1133, 1249, and 1322 HV0.2, respectively, which increased with the increase of Al2O3 mass fractions, the enriched zones of Al2O3 played a major role in the strengthened coatings. 3.4. Oxidation resistance TGA was used to investigate the oxidation behaviors of CoCrAlYTaSi coatings with the different Al2O3 mass fractions. Fig. 8 shows the results of mass gain per unit as the temperature from 400 up to 600 °C. The total mass gain per unit area of coatings with the Al2O3 mass fractions of 15, 30 and 45% were 33.96, 29.82, and 26.27 mg •cm−2, respectively, indicating that the coatings had better oxidation resistance at the range of 400–1000 °C. The mass gain per unit area of coatings with the Al2O3 mass fraction of 15% kept the fast increase in the heating process, while that of coatings with the mass fraction of 45% increased slowly, suggesting that the oxidation resistance enhanced with the increase of Al2O3 mass fractions. The diffusion coefficient of Al was higher than those of other elements at the grain boundary of Al2O3, which improved the oxidation resistance of coatings. Fig. 9 shows the morphologies of CoCrAlYTaSi coating cross–sections with the different Al2O3 mass fractions after the oxidation test. The oxide layers with the gray-white were dispersed on the cross–sections with the Al2O3 mass fraction of 15 and 30%, the cracks existed on the oxide layers, which were diffused from the bonding line to the substrate; while the cross–section with the Al2O3 mass fraction of 45% was compact and regular plate structure. Research reported that the Al
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Fig. 4. Morphologies of CoCrAlYTaSi coating surfaces with different Al2O3 mass fractions.
was high enough to form the Al layer at the oxidation beginning [23], however, the rapid consumption of Al led to the decrease of Al at the bonding interface, the Al2O3 reduced to below the critical level. At this time, the spinel oxides of Co, Cr and Y and the Ta-riched oxide through the Al2O3 layer, prevented and slowed down the oxide growth of CoCrAlYTaSi coating with the Al2O3 mass fraction of 45% [24].
3.5. Friction–wear behaviors 3.5.1. COFs and wear rates Fig. 10 (a) shows the COFs of CoCrAlYTaSi coatings with the different Al2O3 mass fractions vs wear time. The average COFs of CoCrAlYTaSi coating with the Al2O3 mass fractions of 0% was 1.12, while those of CoCrAlYTaSi coatings with the Al2O3 mass fractions of 15, 30
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3.5.2. XRD analysis and wear mechanism Fig. 12 (a) shows the XRD analysis of worn track on the CoCrAlYTaSi coating with the Al2O3 mass fraction of 15%. The CoO peak was detected at 36.68 and 42.346°, and the α–Al2O3 peak appeared at 43.173 o, with no γ–Al2O3, this was because the γ–Al2O3 was transformed into the α–Al2O3 after the wear test [32]. The Ta2O5 and Cr2O3 oxides were detected at 36.914°, and the Ti oxide was detected at 42.611°, which was diffused from the substrate into the coating. The above oxides were mainly produced by the wear debris in the tribochemical reaction, when the content of fragments reached a certain value, they were sintered and fixed on the worn track, acting as a lubricant in the wear process to reduce the COFs. The phase of Al9Co2 presented a widened peak, indicating that the grain size was refined. In additional, the TaC, Al13Co4, Cr3C2 and phases were also detected on the worn track. Fig. 12 (b) shows the XRD analysis of worn track on the CoCrAlYTaSi coating with the Al2O3 mass fraction of 30%. The oxides of CoO, Ta2O5 and Ti were detected on the worn track, the crystallization peaks of CoO and Ta2O5 decreased; while that of Al2O3 peak increased. As a result, the thickness of oxide films sintered decreased, more Al2O3 was distributed on the worn track, the oxide films were still acted as a lubricant to play an anti–wear role [33]. Fig. 12 (c) shows the XRD analysis of worn track on the CoCrAlYTaSi/Al2O3 coating with the Al2O3 mass fraction of 45%. The crystalline peaks of CoO and Ta2O5 further decreased due to the reduction of CoCrAlYTaSi mass fraction, only a thin oxide film was formed on the worn track, the hard phases of Al2O3 and Cr3C2 played a dominant role, the COFs were the largest among the three kind of coatings, the hard phase was the main factor of improving the wear resistance [34]. Fig. 13 (a) shows the morphologies of worn tracks on the coating with the Al2O3 mass fraction of 15%. The scratches and parallel furrows were observed on the worn track, the dominant characteristic was adhesive and delamination wear. After further enlarging the plough groove, it can be seen that there was the serious tearing phenomenon along the sliding direction, the debris did not fall off from the coating and continued to wear on the scratched surface, the wear mechanism was abrasive wear. This was because the shearing stress was less than the bonding strength of coating, the shearing failure occurred on the worn track with the wide and deep scratches. Remarkably, the mixed oxides of CoO, Cr2O3 and Ta2O5 oxides were detected on the worn tracks, which played a role of lubrication to reduce the COFs and wear rates. Among them, the Cr2O3 was devoted to enhance the wear performance due to its inherent hardness and chemical stability [35], the wear mechanism of CoCrAlYTaSi coating with the Al2O3 mass fraction of 15% was transited from the adhesive and delamination wear to abrasive and tribo-oxidation wear [36]. Fig. 13 (b) shows the morphologies of worn tracks on the coating with the Al2O3 mass fraction of 30%. The fine abrasive scratches were observed on the smooth worn track, the wear mechanism was micro–cutting, showing the higher wear resistance than the coating with the Al2O3 mass fraction of 15%. The wear rate was [37].
Fig. 5. XRD patterns of CoCrAlYTaSi coatings with different Al2O3 mass fractions.
and 45% were 0.17, 0.23, and 0.35, respectively, decreased by 85, 79 and 69% than the CoCrAlYTaSi coating, respectively. The COFs increased with the increase of Al2O3 mass fractions, this was because the separation of Al2O3 hard phase felled on the worn track under the action of tribo–pair, with no additive liquid and solid lubricants, the average COFs were less than 0.4. Fig. 10 (b) shows the profile curves of worn tracks on the CoCrAlYTaSi coatings with different Al2O3 mass fractions. The depths of worn track with the Al2O3 mass fractions of 0, 15, 30 and 45% were 12, 3.83, 5.53, and 5.92 μm, respectively, and the corresponding wear rates were 1.59 × 10−4, 1.02 × 10−4, 0.54 × 10−4, and 0.3 × 10−4 mm3/N • m, respectively. The profile curves of CoCrAlYTaSi coating with the different Al2O3 mass fractions were more fluctuant than that of CoCrAlYTaSi coating, which indicated that the wear resistance was improved by the addition of Al2O3. The wear rate of CoCrAlYTaSi coating with the Al2O3 mass fractions of 15% was similar to that of laser cladded NiCoCrAlY coating under the same environment [25]; while those with the Al2O3 mass fractions of 30 and 45% were lower than 0.96 × 10−4 mm3 N−1•m−1 of HVOF sprayed NiCoCrAlY–Al2O3 coating [26]. It was remarkable that the COFs of CoCrAlYTaSi coatings with the different Al2O3 mass fractions were significantly lowed than that of CoCrAlYTaSi coating. The oxides appeared on the worn track, which reduced the COFs [27], as shown in Fig. 11. The Al2O3 was the main component of debris on the worn track in the normal wear period, the COFs were stable due to the lubrication of Al2O3, which reduced the friction force between the coatings and the tribo-pair [28,29]. This was also the reason why the CoCrAlYTaSi coating with the different Al2O3 mass fractions took less time than the CoCrAlYTaSi coating from the running-in to normal wear periods [30]. The COFs of CoCrAlYTaSi coating increased with the increase of Al2O3 mass fractions, exhibiting the friction performance of coating with the Al2O3 mass fractions of 15% was the best among the three kinds of coatings. The phases of CoO and TaO played an important role in the formation of continuous lubrication film on the worn tracks [31], which decreased with the increase of Al2O3 mass fractions.
W∝(KIC)−3/4 ×H−1/2
(2)
where KIC was the fracture toughness; and H was the micro-hardness of coatings. From Eq. (2), it can been known the wear resistance improved with
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Fig. 6. Morphologies of CoCrAlYTaSi coating cross–sections with different Al2O3 mass fractions.
the increase of hardness. After adding more Al2O3 content, the abrasive wear was improved due to its higher hardness, which prevented the plastic deformation and adhesion wear occur on the worn tracks. Fig. 13 (c) shows the morphologies of worn tracks on the coating with the Al2O3 mass fraction of 45%. There were layered and scaly features on the worn track, suggesting that the coating was exfoliated and fractured, which originated from the Ti diffusion in the substrate,
the thermal stress at the coating-substrate interface was the main factor of mechanical damage [38]. In this case, the plastic deformation of coating surface was introduced into the grain interior, which resulted in the tensile stress increasing and accumulating along the grain boundary and. In multiphase composites consisting of CoCrAlYTaSi and Al2O3, the internal residual stress was mainly caused by the following thermal expansion mismatch.
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The stress intensity factor was K(t)=Y(σD(t)+q)bd1/2
(4)
where Y and b were the crack geometry parameters; σD(t) was the tensile stress during the plastic deformation damage; q was the thermal expansion mismatch; d was the grain size [39]. From Eq. (4), the increase rate of stress intensity factor K(t) was faster than that of thermal expansion mismatch q with the increase of Al2O3 mass fractions. This was because the increase of Al2O3 hard phase at the interface decreased the metallurgical bonding strength between the coating and the substrate, which led to the brittleness increase of coating. By magnifying the fracture surface, it was found that the morphology presented the brittle fracture with the relatively flat, only a small amount of micro–abrasive grinding. It was included that the increase of Al2O3 mass fraction improved the abrasive resistance, the brittle fracture produced the micro–grinding effect on the worn track, and the wear resistance of coating with the Al2O3 mass fraction of 45% was excellent. However, the coating easily broken and fell off to increase the COFs with the increase of Al2O3 mass fractions [40], which reduced its friction performance. From the above analyses, it can be seen that the wear mechanisms of CoCrAlYTaSi coatings with the Al2O3 mass fractions of 15 and 30% were abrasive wear and micro–cutting, respectively; while that with the Al2O3 mass fraction of 45% was micro–abrasive wear, the main factor was the increase of the Al2O3 mass fractions to reduce the wear degree.
Fig. 7. Hardness of CoCrAlYTaSi coating cross–sections with different Al2O3 mass fractions.
4. Conclusions (1) The dendritic structure of CoCrAlYTaSi coating with the Al2O3 mass fraction of 15% is further refined, which grows perpendicular to the substrate, the metallurgical bonding is formed at the coating interface. (2) The hardness of CoCrAlYTaSi coatings the Al2O3 mass fractions of 15, 30 and 45% is 1133, 1249, and 1322 HV0.2, respectively, which increases with the increase of Al2O3 mass fractions. (3) The total mass gains per unit area of CoCrAlYTaSi coatings with the Al2O3 mass fractions of 15, 30 and 45% is 33.96, 29.82, and 26.27 mg/cm2, respectively, the oxidation resistance in the range of 400–1000 °C is excellent. (4) The average COFs of CoCrAlYTaSi coatings with the Al2O3 mass fractions of 0, 15, 30 and 45% are 1.12, 0.17, 0.23, and 0.35, respectively, and the corresponding wear rates are 1.02 × 10−4, 0.54 × 10−4, and 0.3 × 10−4 mm3/N•m, respectively. The wear mechanism is abrasive wear and micro–cutting, the wear reduction is the best among the three kinds of coatings.
Fig. 8. Mass gain per unit area of CoCrAlYTaSi coatings with different Al2O3 mass fractions in range of 400–1000 °C.
q = V(1-V)E△T△α
(3)
where V was the volume fraction of second phase Al2O3; E was the average elastic modulus; △T was the range of temperature; and △α was the thermal expansion mismatch.
Fig. 9. Microstructure of CoCrAlYTaSi coating cross–sections with different Al2O3 mass fractions after oxidation test.
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Fig. 10. COFs vs wear time and profiles of worn tracks on CoCrAlYTaSi coatings with different Al2O3 mass fractions.
Fig. 11. Wear model of CoCrAlYTaSi coatings with different Al2O3 mass fractions wear.
Fig. 12. XRD analysis of worn tracks on CoCrAlYTaSi coatings with different Al2O3 mass fractions.
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Fig. 13. Morphologies of wear tracks on CoCrAlYTaSi coatings with different Al2O3 mass fractions.
Acknowledgements
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